Sooo, update on this topic (maybe someone finds it useful). I tested it and it seems that it really needs to be packed horizontally and kept that way during travelling. Otherwise, the paints may spill and mix. Though I really must mention two things: mine is bought on Alliexpress (it may be some kind of copy) and I did thin my gouache paints with blending medium to keep them from drying. However, the palette really does its job when it comes to keep paints fresh.